1. Field of the Invention
The present invention relates to a monitor optical device for introducing part of light beam emitted from the light source to a detector for monitor and an optical pickup apparatus.
2. Description of the Related Art
In fields of the optical pickup and printer, often, auto power control (APC) has been adopted, in which laser beam emitted from a rear end face of a laser light source is monitored and the quantity (intensity) of the emitted light is controlled corresponding to its result in order to stabilize the quantity of emitted light of the laser light source. However, this method is difficult to control the quantity of light accurately by feedback because the quantity of light of laser beam emitted from both the front side and rear side of the laser light source are different. Further, because beam is emitted to an opposite side (rearward) to a predetermined direction (forward), the quantity of emitted light of the laser light source cannot be used effectively.
Thus, in recent years, front monitor type, in which laser beam emitted forward from the laser light source is branched partially at a predetermined position of a light recording optical system and the branched partial laser beam is used as monitor beam, has been often used.
As such a method, for example, (1) using part of emitted light as monitor beam by inserting a mirror in which reflectivity corresponding to wavelength is set in part of an optical path, (2) by reflecting light impinging on the surrounding of a prism or a mirror which multiplexes lights for CD, DVD or the like by several percentages, using it as monitor beam (see for example, U.S. Published application No. 2003/218952 A1, FIG. 1), and (3) by disposing diffraction grating in an optical path and branching light by diffraction using hologram, securing monitor beam (see, for example, Japanese Patent Application Laid-Open No. 8-279174).
According to the structure for picking out monitor light by disposing the light beam on an optical path as mentioned in (1), (3), the number of components is increased because such an optical device is needed. Particularly, in recent years, an apparatus in which a next generation optical disk meeting a blue laser and such optical disk as existing CD and DVD are available in the same optical system has been proposed. However, if the above-described methods (1) and (3) are applied to this kind of the apparatus, the monitor light needs to be picked out for each wavelength and thus, the aforementioned optical device is needed corresponding to each wavelength. As a result, the number of components increases, thereby inducing complexity of the apparatus and increase in manufacturing cost.
If in a mirror (particularly dielectric mirror) in which reflectivity corresponding to the wavelength is set up, the wavelength of laser beam impinging on the mirror deviates due to fluctuation at the time of manufacturing of a laser light source or changes in temperature, the quantity of reflected light on the mirror or the quantity of monitor beam changes accompanied by that. Therefore, if there is fluctuation of laser light source or changes in temperature, the method (1) is incapable of executing appropriate control on the quantity of light based on the monitor beam.
If the power (quantity of emitted light) of laser beam is changed or there is a change in temperature, the spreading (radiation angle) of laser beam emitted from the light source is changed. Thus, according to the method (2) using light impinging on the prism or the surrounding portion of the mirror as monitor light, the quantity of monitor beam is likely to change due to influences of change in power, so that appropriate control on the quantity of light cannot be executed based on the monitor light.